In the field of a steering device for a vehicle such as an automobile, it has been well known to reduce steering burden of a driver and improve steering feeling by controlling assist torque of an electric power steering device or by controlling steering reactive force of a steering device of steer-by-wire-type.
Particularly in Japanese Patent Application Laid-Open (kokai) No. 2006-182052, a steering device of steer-by-wire-type is described which controls steering reactive force in accordance with steering torque. In view of the fact that steering reactive force lowers in the specific steering frequency area, the steering device suppresses lowering of steering reactive force in a specific steering frequency area by reducing a gain of a transfer function from steering angle to steering torque.
In Japanese Patent Application Laid-Open (kokai) No. 2003-306158, an electric power steering device is described which conducts damping control (viscosity compensation control) on the basis of steering angular velocity. The electric power steering device enhances the component of around yaw rate resonance frequency contained in steering angular velocity signal to thereby suppress lowering of steering reactive force when steering frequency is at or near yaw rate resonance frequency.
In general, referring to the response of steering reactive force against steering angle, magnitude of steering reactive force lowers at a specific steering frequency and the phase of steering reactive force varies in accordance with steering frequency. Particularly, in an area where steering frequency is higher than the specific steering frequency, the phase of steering reactive force advances, so that damping property increases and viscous feeling sensed by a driver rises. At the same time, in an area where steering frequency is lower than the specific steering frequency, phase of steering reactive force delays, so that damping property decreases and viscous feeling sensed by a driver lowers.
In conventional steering devices such as those described in the above references, it is not considered that, the phase of steering reactive force response against steering angle varies in accordance with steering frequency, which results in variation in viscous feeling. Accordingly, in conventional steering devices, it is not possible to prevent viscous feeling from varying in accordance with steering frequency, and accordingly there is a room to improve in enhancing steering feeling.